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Re: [T3] too much mind time


Hey Jim- 

> I've been really busy lately and haven't had time to look at things 
> like this in detail. Thus this has taken a back seat until I could 
> work thru it and see where the disagreements really were. 

No prob. 


> This is where I made my really silly mistake.The 330 is Hertz, not 
> RPM, so I have to multiply by 60 to get RPM, which converts to 
> 19,800 rpm. Some of you may recall that I expressed surprise at 
> how low my first number came out. It CERTAINLY was, a factor of 
> 60 makes a BIG difference! 

"Hey... my engine is only spinning at 5. Oh, wait... RPH... hmm" :-) 


> > Yes, but one detail: the pulse doesn't get 720 degrees - it gets 
> > 720 minus the time that the intake valve is open.  I don't have the 
> > specs for the stock cam offhand, but I'm guessing that it's about 
> > 210 degrees at 0.050" lift. 
> 
> > The total timescale goes from x=0 to x=A.  A=[(720-ECD)/360]*[60/W] 
> > where ECD is the effective cam duration in degrees, W is the engine 
> > speed in RPM, and x is in seconds. 
> 
> > A quarter-wave is one travel through the runner.  So, T=4*L/V where T 
> > is the period in seconds, L is the effective runner length (i.e. 
> > runner length plus port length plus 1/2 the diameter of the runner) in 
> > inches, and V is the speed of sound in hot air in inches per second. 
> 
> The intake air should be cooler than ambient after expanding 
> adiabatically in the throttle body, but I don't think that's important 
> here. 

At full throttle?  Hardly... the plenum pressure should be as close to ambient 
as possible. 

> I understand how you're approaching this and it makes sense 
> to me now. Note that we are actually calculating the SAME thing 
> except that I was using the full period, and you are using only the 
> time from the valve closing to its next opening. I agree that your 
> method is the correct one. 

After looking it over again, you're right - the only differences we have in the 
actual formulae are the timescale and the mean air velocity. 


> > The goal, obviously, is to make that timescale an integral multiple of 
> > the period of the wave.  So, A=N*T where N is our reflection value. 
> > 
> > Result: 
> > V*(720-ECD)=24*W*N*L 
> > 
> > Let's plug this in: 
> > L=20.625 in 
> > V=15000 in/sec 
> > ECD=210 degrees 
> > 
> > For N=1, W=15455rpm.  For N=2, W=7727rpm.  For N=3, W=5151rpm. 
> 
> It took me awhile to figure out how you got there, but I finally 
> managed to duplicate it: 

<respectfully snipped... and your call of using w instead of W and t instead of 
A is a bit cleaner :-)> 

> V*(720-ECD) = 6*4*w*N*L = 24*w*N*L 
> 
> This allows us to use the "subharmonics" of the natural frequency 
> of the pipe to get down into a reasonable operating range for the 
> engine, just as you explained. 
> 
> Just as a curiosity, I wonder if there is ever any application of the 
> true harmonics which would come out to: 
> 
> V*[(720*N)-ECD] = 24*w*L 
> 
> I think only the odd harmonics would work here, because the even 
> ones would hit an open valve on an intermediate bounce. Note that 
> for large values of N, this approaches the numbers I started out 
> with, or meant to, if I had only made the proper conversion from Hz 
> to RPM.. I think these resonances would be much stronger, but 
> they only occur at impractically high RPMs. 

The problem I see with them is that that we are setting up these standing waves 
from a sudden pulse caused by the closing of the valve in an otherwise still 
(ideally, of course) airmass.  However, if we want to have the wave last for 
multiple cycles, that wave will have to survive an intake pulse, which I 
seriously doubt it can.  That would require the exact same standing wave to 
persist though an accelerating and decelerating airmass - one we really can't 
idealize as still. 

To realize the magnitude of the air that suddenly is sucked in compared to the 
intake system, let's consider the 1585cc T3 engine running at 80% volumetric 
efficiency.  That is 19.345 in^3 per intake cycle.  Let's assume that the 1.25" 
diameter is roughly constant throughout the heads and runners.  Then, the 
cross-sectional area of this is 1.2272 in^2.  That equates to a length of 
15.763", or about 79% of the ~20in runners.  The wave will die... 


> > We can also consider when the resonance effect decreases performance. 
> > This corresponds to the valleys of the cosine wave, when N=1.5, 2.5, 
> > and 3.5.  Those correspond to 10303rpm, 6182rpm, and 4416rpm.  That 
> > last one is interesting: the -3db range of that valley is from about 
> > 4263rpm-4579rpm. Here there is a DECREASE in performance thanks to the 
> > runners, which is where the stock 1600 T3 engine has its peak. 
> > Between this and the unequal-length runners, I don't think that the VW 
> > engineers were too concerned with resonance when they designed this 
> > system :-) 
> 
> That's not at all clear to me. It seems to me that this is exactly 
> what you would want for a street car, which is what they were 
> designing, after all. By placing an intake resonant depression right 
> on top of the other peaks they were thereby placing the resonant 
> peaks just to either side of the other peaks. The result would have 
> been to broaden the power band at the expense of the peak power, 
> but this is a desirable thing in a street car. 

That strategy is not employed by engine designers.  If we break it up, then the 
runner length is to set the peak point.  Then, a properly-sized plenum and 
runner diameter (just large enough to do what you want, but no more!) complete 
the package, rounding out the sharp peak.  The old V8's often times used the 
second reflection - it allows for the broadest powerband and the least 
dampening from reflections.  The european cars often used the third reflection 
because it fits much more nicely into the engine compartment.  Plus, with a V6, 
other effects occured that further enhanced the effect, so there wasn't excatly 
a loss with the extra reflection.  I don't know of any cases that used the 4th 
reflection. 

If you look at a dyno readout of a stock engine, then you'll find that right 
after the 4000-4500rpm peak, power steeply declines.  If VW did actually do 
this intentionally (which I tend to doubt they did), then they wouldn't have 
put a peak where a) there isn't any power to start with and b) the engine falls 
apart (a stock T1/T3 actually designed purposely to do 5200rpm routinely won't 
last very long). 


> You should also just forget about the unequal-length runner 
> business; they differ by only 1.5%. 

Just enough to make it run unevenly... 

But, I argue again, it doesn't matter.  If VW really did want to do this, 
they'd have focused on tuning the exhaust first, intake second.  The engine 
already runs slightly unevenly due to the exhaust so the intake bit wouldn't 
matter.  VW's strategy was to limit performance to increase longevity in an 
idiotproof way.  This is cheaper and reduces complaints from customers with 
well-tuned 1600's doing 80hp that have their engine's fry in a short amount of 
time. 

> > You're correct.  Q=A*V=VOL*VE*T, where flow is Q, area is A, velocity 
> > is V, cylinder volume is VOL, volumetric efficiency is VE, and time 
> > (i.e. the amount of time the intake valve is open) is 
> > T=[ECD/360]*[60/RPM]. 
> 
> There's a problem here, in that I can't get the dimensions of the 3 
> parts of your definition to agree. If we use English units, 
> 
> Q is in in**3/sec 
> A*V is in**3/sec, but 
> Vol*VE*T is in**3*sec 
> 
> The more I think about this, the more I think it was just a simple 
> typo, and it should read Q=A*V=Vol*VE/T. Is that what you 
> intended? 

Yes indeed.  Just a silly typo... sorry! 


> I checked, and VE is just defined as VE= Q1/Q2=Q1/(d*Vol), 
> where Q1 is the mass of the air that actually gets in the cylinder, d 
> is the air density, and Vol is the displacement of the cylinder. 
> There are different ways of calculating Q2 depending on whether 
> you use atmospheric pressure or the pressure outside the intake 
> valve to calculate the air density. Different definitions are used 
> depending on what you're interested in. 

There are different methods of determination (including using some numbers and 
a dyno plot of torque... although the cleanest is probably with a mass air flow 
sensor from an FI), but the basic definition is the same: all other things 
equal, how much air in comparison to the displacement can the cylinder suck in 
on the intake stroke?  Many things can effect it, including adequate exhaust 
scavenging.  And, of course, other methods of aspiration... :-) 

Take care, 
Shad 

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